In most known thermal forming methods (see U.S. Pat. No. 3,115,677), a previously manufactured and completely cooled and stored thermoplastic web is heated, usually by infra-red radiation, prior to the thermal shaping on one or both surfaces, to a forming temperature. Thermoplastic materials being poor heat conductors however, the web must be raised to very high and usually uncontrollable temperatures at the surface or surfaces in order to supply sufficient heat to raise the web core to the desired temperature. If relatively thick webs, for example webs exceeding 1 mm in thicknesses, are to be formed to articles, considerable time is required for heating the web through the surfaces. Furthermore, considerable damage may be caused at the web surfaces, whether thick or thin webs, because of excessively high temperatures and especially because of possible oxidation at those high temperatures, so that thermoplastic articles so made may be degraded at least at the outer surfaces.
In-line thermal forming methods are known (German Auslegeschrift No. 1,165,241; U.S. Pat. No. 2,891,980), wherein a thin thermoplastic web is continuously cast from an extruder followed by a fish-tail nozzle and is fed to a device converting the continuous advance into a stepwise advance, whence the web will be brought to negative (female) dies in a thermal shaping machine. In these known methods, the web is set for temperatures as low as possible during casting from the nozzle so that it may be immediately fed to the advance converter. The heat supplied to the web during extrusion is kept so low that it usually will not suffice or be retained for the ensuing thermal forming, and so additional heaters acting on the web surface are provided which may result in the above described drawbacks with respect to web surface.
Another example of prior art methods and apparatus involving successive web forming and thermal shaping is disclosed in U.S. Pat. No. 2,697,328.